How to remove the dark, sharp outlines in this water splash simulation?
As you can see in the screenshot, this happens for the drops that aren't round, but rather triangular. If you set the default cube to smooth and give it a glas material it will look similar. There is not enough geometry to support proper transmission behaviour. 3 solutions.
Set it to auto-smooth. This is a hack, while it will save you RAM it's bound to flicker in an animation and might cause strange artefacts, worth a shot though.
Use a subsurf modifier after the water, quick fix, will increase render times.
Increase the simulation detail. The latter will increase your simulation time and render time, but also will yield more precise results.